A variety of equipment and techniques have been developed to assist manufacturers of integrated circuits for testing those circuits while still in the form of dies on semiconductor wafers. In order to quickly and selectively electrically interconnect metalized contact pads (also known as “bonding pads”) on each die to electrical test equipment (known as a “prober machine”), arrays of slender wires or other contact media are provided. The contact media are arranged on conventional printed circuit boards so as to be positionable on the metalized contact pads associated with each semiconductor die. As is well known by those of ordinary skill in the art, those printed circuit board test cards have come to be known as “probe cards” or “probe array cards”, and the contact media have come to be known as “probe card pins” or “probe pins” or “probe wires”.
As the component density of semiconductor circuits has increased, the number of contact pads associated with each die has increased. It is now not uncommon for a single die to have upwards of 600 pads electrically associated with each die. The metalized pads themselves may have as little as a ten μm gap there between with an on-center spacing on the order of 50 μm to 100 μm. As a result, the slender probe wires of the probe array cards have become much more densely packed. It is highly desirable that the free ends or “tips” of the probes be aligned in a common horizontal plane, as well as have the proper positioning with respect to one another within the plane so that when the probes are pressed down onto the metalized pads of an integrated circuit die by a prober machine, the probes touch down substantially simultaneously, and with equal force while being on target. As used herein, the terms “touchdown”, “rest” and “first contact” have the same meaning. In the process of making electrical contact with the pads, the probes are “over traveled” causing the probes to deflect from their rest position. This movement is termed “scrub” and must be taken into account in determining whether the rest position and the over travel position of the probes are within specification for the probe card.
The assignee of the present invention has developed equipment for testing the electrical characteristics, planarity and horizontal alignment, as well as scrub characteristics of various probe cards and sells such equipment under its Precision Point™ line of probe card array testing and rework stations. A significant component of these stations is a planar working surface known as a “check plate”. A check plate simulates the semiconductor die undergoing a test by a probe card while checking the above described characteristics of the probes. A suitable check plate for use with the assignee's Precision Point™ equipment is described in detail in U.S. Pat. No. 4,918,374 to Stewart et al. issued Apr. 17, 1990, the disclosure of which is incorporated herein by reference. It is sufficient for the purposes of this disclosure to reiterate that while the subject probe card is held in a fixed position the check plate is moved horizontally in steps when testing the horizontal relative positioning, and vertically in steps when testing the touchdown contact and over travel position of each probe tip. Previously, and as described in the above-identified patent, horizontal position information for each probe tip was determined by translating an isolated probe tip in steps across resistive discontinuities on the check plate. In recent years, this technique has been altered by placing a transparent, optical window in the surface contact plane of the check plate with a sufficiently large surface dimension so as to permit a probe tip to reside thereon. An electronic camera viewing the probe tip through the window digitizes the initial touch down image of the probe, and a displaced position of the probes due to “scrub” as the check plate is raised to “over travel” the probe. The initial touch down position is compared to the anticipated touch down position to assist an operator in realigning that particular probe.
Another prior art technique for determining relative probe tip positions in a horizontal (e.g. X-Y) plane is described in U.S. Pat. No. 5,657,394 to Schwartz et al., the disclosure of which is incorporated herein by reference. The system disclosed therein employs a precision movement stage for positioning a video camera into a known position for viewing probe points through an optical window. Analysis of the video image and the stage position information are used to determine the relative positions of the probe points. In systems of this type, a “reference” probe position is determined primarily through information from the video camera, combined with position information from the precision stage. If the pitch of the probes on the probe card is small enough, two or more probes can be simultaneously imaged with the video camera. The position of this adjacent probe is then referenced with respect to the “reference” probe from information from the video camera only. The camera is then moved to a third probe, adjacent to the second probe and this process is repeated until each probe on the entire probe card has been imaged.
In addition to the above devices for measuring various parameters of probe cards, equipment is available for measuring actual “scrub marks” made by probe card pins on a test wafer which has been impressed by the probe card with a prober machine. One such apparatus is manufactured by Visioneering Research Laboratory, Inc., Las Cruces, N.Mex. to provide high quality imaging of scrub marks made by a probe card and a prober machine. It is well known that scrub patterns analyzed by a probe card analysis machine do not match the scrub marks produced on a test wafer imaged by a scrub mark analysis machine. The test wafer models the surface characteristics of bonding pads on a semiconductor die. As stated above, the measurement surface on the probe 15 card analyzer is typically manufactured from hardened steel, or more recently a transparent synthetic or natural crystal such as sapphire. This probe card analysis testing surface is much harder than the aluminized surface of a semiconductor bonding pad. The typical annealed aluminum surface of a semiconductor bonding pad in fact yields under pressures applied by the semiconductor probing machine which may be on the order of 5 grams per pin. Remembering that the pin surface is very small, the pressure applied is sufficient to break the surface of the aluminum bonding pad causing the probe tip to ‘dig in’ during probe pin overtravel. Within a short distance, the tip of the probe pin plows so deeply into the aluminum surface that it stops even though the probe card continues its downward travel. This phenomenon has been characterized as “stubbing” by the assignee of the present invention. In contrast, the hard metal or sapphire surface of the probe card analysis machine does not yield under pressure from the probe pin. In addition, the metal or sapphire contact surface of the probe card analysis machine is highly polished and has a much lower coefficient of friction than does the aluminized surface of the semiconductor die bonding pad.
As a result, the probe pin does not stub on the probe card analysis machine, and the probe pin tip travels further than it does on the aluminized bonding pad. Furthermore, the place at which the probe pin first contacts an aluminized bonding pad (or the aluminized semiconductor test wafer which simulates the bonding pad in the scrub mark analysis machine) or “touch downs” position of the probe pin is not readily discernable in the scrub mark made in the aluminum surface. The scrub mark resembles a brush stroke with a faint starting position and a deep, clearly defined ending position. Conversely, the probe card analysis machine accurately captures the touch down position of the probe pin on the measuring surface as well as its full travel across the surface without stubbing. Therefore, neither the touch down position, nor the end of travel position of the probe pin on the probe card analysis machine, matches corresponding positions on either an actual aluminum bonding pad or on a semiconductor test wafer imaged by a scrub mark analysis machine.